This patch also introduces testing for libomptarget-nvptx
which has been missing until now. I propose to add tests for
all bugs that are fixed in the future.
The target check-libomptarget-nvptx is not run by default because

we can't determine if there is a GPU plugged into the system.

it will require the latest Clang compiler. Keeping compatibility with older releases would prevent testing newer code generation developed in trunk.

Considering your comment in the description about requiring latest Clang perhaps you should revisit this patch: D46842

D46842 only helps if the OpenMP runtime is not built stand-alone.
For the OpenMP runtime tests, we take care that the tests also work with other compatible compilers, and also with older versions.

We might configure different behavior for stand-alone and in-llvm-tree builts.
Considering the necessity of a GPU supporting the features, it makes sense to have a separate target for these tests and executing them manually.

Considering your comment in the description about requiring latest Clang perhaps you should revisit this patch: D46842

In my opinion the two matters are different with regard to an important detail:

The linked change in D46842 is about building libomptarget-nvptx with in-tree Clang. As a result the build system would recompile the library whenever there is a change in the Clang sources. For me this drawback outweights the potential gain; in particular you can build the bclib with Clang 7.0 and it will still be inlined when building an application with a later version of Clang (e.g. current trunk), so I don't see the need to use in-tree Clang.

This patch adds testing infrastructure which will kick in after libomptarget-nvptx has already been built. At the moment the tests will only work with the latest Clang because it relies on --libomptarget-nvptx-path and code generation that is not yet included in a released version of LLVM. Incidentally I'm listing this as one of the reasons why check-libomptarget-nvptx is not included in neither check-all nor check-openmp. (However I think "we can't determine if there is a GPU plugged into the system" is the one that really matters and we'll probably never be able to solve this, so I don't see enabling these tests by default anyway.)

I hope this clarifies my thoughts that I've brought up in some review threads. From my point of view there is no contradiction to what I propose in this patch and for me the choice of not enabling the tests by default is the logical consequence of my previous statements.